1. Field of the Invention
The present invention relates to an eye axial length measurement apparatus for measuring an axial length of an examinee's eye (a patient's eye).
2. Description of Related Art
There is known an ultrasonic eye axial length measurement apparatus for measuring an axial length of a patient's eye by making an ultrasonic prove touch a cornea of the patient's eye (see JP2001-187022A for example). Another known one is a non-contact eye axial length measurement apparatus including a measurement optical system arranged to project measuring light to a patient's eye and detect reflection light therefrom and optically measure an axial length of a patient's eye by using an optical interference technique without touching the eye.
A concrete configuration of the non-contact eye axial length measurement apparatus is briefly explained. The measurement optical system includes: an irradiation optical system for splitting light emitted from a low coherent light source to irradiate a cornea and a fundus of a patient's eye; a photo-receiving optical system with a photo-receiving element for photo-receiving interference light generated by reference light and measurement light, one of which is corneal reflection light or fundus reflection light produced by the irradiation optical system and the other is the remainder of them; and an optical path length changing member disposed to be movable into an optical path of the irradiation optical system or the photo-receiving optical system to change an optical path length. Generally, the axial length of the patient's eye is determined based on the moved position of the optical path length changing member when an interference signal is detected by the photo-receiving element (for example, EP1602320A1 (JP 2005-342204A) and US2007/0002277A1 (JP 2007-37984A)). As a system of the measurement optical system, there are known for example a time domain system disclosed in EP '320 and a Fourier domain system disclosed in US '277.
The ultrasonic eye axial length measurement apparatus has an advantage that can measure even a cataract eye. However, eye-drop anesthesia to the patient's eye is required. This is a large burden on a patient and also an additional work for an examiner. On the other hand, the non-contact eye axial length measurement apparatus has an advantage that can perform measurement in non-contact manner without placing a burden on the patient. However, such optical measurement is unsuitable for some patients' eyes. For example, in the case of an eye with severe cataract, measuring light is apt to be blocked by a cloudy portion and it is hard to obtain a high reliable measurement value.